Try driving it with the seats down and see if the noise is still there. If it is, it might not be the back rests rubbing on something.
Nope, VW rather generously didn't bother with sound insulation on the boot floor. It's just the flimsy little semi-rigid mat. I lined my boot floor with self adhesive acoustic pads but it didn't achieve a lot. They're just noisy cars in general by todays standards.